Download Introduction to Thunderstorms, Tornadoes, and Hurricanes All three

Introduction to Thunderstorms, Tornadoes, and Hurricanes
All three storms have:
 High winds
 Rain
 Usually occurs at a certain time of year in certain locations
 They can be forecast, detected, and tracked by the NWS –National Weather Service
Thunderstorms:
 It is a disturbance in the atmosphere that is characterized by lightning and thunder.
 A thunderstorm may have gusty winds, heavy rain, sleet, snow or hail, or no precipitation at all.
 A severe thunderstorm can produce flash floods and tornadoes.
 Thunderstorms generally move in the direction of overhead winds, or in the direction of humid,
unstable conditions.
 Thunderstorms can occur in any location, but are generally in the middle latitudes.
 In the Southeastern US, they occur most often along the Gulf of Mexico, especially in Florida,
which have afternoon thunderstorms regularly in summer.
 Most thunderstorms occur in the spring and summer months; during the warmest part of the
day when warm air is most likely to be in motion.
 Other thunderstorms in the Central Plains may occur at night.
 Thunderstorms occur in warm air masses and along fronts.
 Warm air can be forced upward where it cools and condensation occurs, forming cumulonimbus
clouds that can reach heights of 18 km.
 When rising air cools, water vapor condenses into water droplets of ice crystal.
 Sinking rain, cooled air and strong updrafts of warmer air causes the strong updrafts of warmer
air causes the strong winds associated with thunderstorms.
 Heavy rain for a period of time can cause flash flooding which is dangerous because it can occur
without warning.
 A well -developed thunderstorm can cover an area as large as 5 to 10 square miles.
 During thunderstorms you often see thunder clouds, or cumulonimbus clouds in the sky.
 If the temperature in part of a thundercloud falls below freezing and winds are strong, the
raindrops can develop into hail.
 It is rare, but a thunderstorm can occur during winter and may have snow as precipitation.
 This type storm is called a “thundersnow”.
 In a fraction of a second, a lightning bolt can discharge as much energy as a medium-sized
nuclear reactor with currents up to 160,000 amperes.
 At any given moment, an estimated 1500 to 2000 thunderstorms are occurring on Earth.
 These storms can trigger 6000 lightning flashes per minute.
 The sound of thunder travels more slowly than the lightning flash from a thunderstorm.
 An observer will see the flash of lightning long before hearing the sound of thunder.
Tornado:
A tornado is a rotating column of air that forms from thunderstorms over dry land when warm moist air
meets cool dry air head on.
 Hurricanes can also cause tornadoes.
 The United States has more tornadoes than any country in the world.
 Most tornadoes occur in North America.
Most tornadoes in North America occur in “Tornado Alley”, in the western plains, and in “Dixie
Alley”, in the lower Mississippi Valley, between Texas and the Gulf states and Tennessee.
 About ¾ of all tornadoes in the US develop between March and July, during late afternoon.
 Most tornadoes occur during the month of May.’ averaging about 5 per day.
 The most violent tornadoes occur in April.
 The diameter of most tornadoes is between 100 and 600 meters (328-1969 feet).
 Some tornadoes stand almost still, while others move at speeds faster than 62 miles per hour
(100 km).
 There is no way to predict the exact path of a tornado, but meteorologists can estimate where
it will go within a band several miles wide.
 A tornado’s vortex contains swirling winds that can move up to 350 km (217 miles) per hour.
 Waterspouts are tornadoes that form over water.
 Much of a tornado’s destructive power comes from its strong winds which can lift heavy objects
and turn them into missiles.
 Japanese-born meteorologist Tetsuya “Ted” Fujita, who was known as “Mr. Tornado”,
developed the Fujita scale for measuring tornadoes damage.
 In 2007 the Fujita scale was revised, it is now the Enhanced Fujita Scale of Tornadoes
Hurricanes:
 Hurricanes are massive, rotating storms, formed when warm moist air rises over tropical waters.
 A rotating storm is called a hurricane when it forms north of the equator in the Atlantic and
eastern Pacific Oceans.
 It is called a typhoon when it occurs north of the equator in the western Pacific Ocean.
 It is called a cyclone when it forms in the Indian Ocean and off the coast of Australia.
 A hurricane can vary in size. Typhoon Tip had a diameter of 1100 km (684 miles)
 Tropical Cyclone Tracy had a diameter of 100 km (62 miles) when it struck Australia.
 Hurricanes can move at speeds of 8 to 24 kilometers (5 to 15 miles) per hour.
 Hurricanes begin as tropical storms, which have wind speeds of 64 to 118 (40-73 miles) per
hour.
 Hurricanes have wind speeds of 119 to 250 km (74-155 miles) per hour or more.
 US engineer Herbert Saffir and meteorologist Bob Simpson developed a scale that uses wind
speed and storm surge to measure damage.
 A hurricane rarely becomes more powerful as it hits land because it gets most of its energy from
the warm ocean water beneath it.
 A hurricane can last a week or longer and several can occur at the same time, naming them
reduces confusion.
 Names are selected in advance by governments of the different regions.
Notes from class: Understanding Weather and Climate, STC. AMSTI text, pages 30-37